The present invention is directed to a method and apparatus for sensing the clearance of fault current on an ac transmission line.
In power transmission systems the growth of fault current levels has been nearly geometric. Because of the large magnitude of the fault current, the "brute force" solution of using ever larger circuit breakers no longer works; the largest breakers available will not meet anticipated interrupting requirements.
One solution is a fault current limiter which is series connected in the transmission line and reduces fault current to a low enough level so that normal circuit breakers can be used. Such limiters are discussed in an article entitled "Fault Current Limiters: Problems and Prospects" in EPRI Journal (Electric Power Research Institute), February 1976, pp. 14-19 by Richard Kennon.
In fault current limiting a basic problem is the reliable determination that a fault in a particular power circuit has been effectively cleared. This determination must be made as quickly as possible so that necessary electro-mechanical devices such as a fault current limiter bypass switch can be activated. Specifically, the fault current limiter is momentarily dissipating megawatts of energy and thus must be quickly shunted after the fault condition has been cleared.
The most difficult sensing discrimination occurs when the placement of the fault current limiter in the power system allows normal load current to continue to flow immediately after fault current interruption. This prohibits the use of any type of "zero current" sensing technique.